Method for fabricating thermally insulated pipeline section

ABSTRACT

Prefabricated insulated pipeline section having a one-piece cast cured and dried insulative liner formed of lightweight aggregate in a foamed portland cement paste binder bonded to the inner surface of a tubular outer casing. The liner composition may be cast in place between the liner and the pipe, and wet cured at elevated temperature. Bonding between the pipe and the liner composition is avoided by employing a cement cure retarder on the outer surface of the pipe or by mechanically debonding the pipe from the liner during curing. After curing, the pipe is withdrawn to expose the inner surface of the liner to permit rapid drying at elevated temperature. The prefabricated insulated sections permit simplified pipeline construction at reduced cost as compared with traditional methods of applying half shells of insulation to the pipeline on site.

This application is a division of application Ser. No. 681,277, filedDec. 13, 1984, now U.S. Pat. No. 4,640,312 issued Feb. 3, 1984.

The present invention relates to prefabricated pipeline sectionsespecially, although not exclusively, suitable for use in conveying hightemperature fluids, e.g., steam at temperatures in excess of about 400°F.

Conventionally, in the construction of thermally insulated pipelinesintended for high temperature service, relatively short lengths ofsemi-cylindrical calcium silicate insulation material are applied to thepipeline on site after the individual lengths of pipe have been weldedtogether. The short lengths of insulation material, known as "halfshells" are attached to the pipe by strapping and a protective orweather-resistant covering is applied. This construction procedure is,however, time-consuming and expensive.

Considerable economies could be effected by the in-plant manufacture ofprefabricated insulated pipeline sections which could then betransported to the location of the pipeline installation for joiningtogether to form the completed pipeline. The conventional calciumsilicate material, however presents the problem that its curing reactionis attended by considerable shrinkage of the material, and theprefabrication of calcium silicate sections beyond a certain sizebecomes impracticable, as the increased length of the sectionsexacerbates the shrinkage problem and tends to result in the finalproduct being defective due to the presence of numerous voids andcracks. Pre-assembly of individual half shell sections onto the pipe inthe factory would be possible, but the half shell sections would beprone to loosening or shifting during transport, and the costs would notbe significantly reduced as compared with the costs of the traditionalon-site assembly method.

We have now developed a prefabricated thermally insulated pipelinesection employing an insulating composition based on portland cement.One advantage of this is that it is more cost effective than the calciumsilicate material, and, further, the curing of portland cement is notattended by the shrinkage problems that result with calcium silicate. Wehave found that it is necessary to dry the cured cement-based insulationmaterial prior to use. Usually, substantially all free moistureremaining after completion of the hydration reaction has to beeliminated from the cured cement material. The presence of anysubstantial amount of free moisture in the insulation material givesrise to such problems as the thermal conductivity of the insulationmaterial being excessively high, as the water is a relatively good heatconductor. Further, it is normally desired to encase the insulationmaterial with a moisture-impermeable casing. At high servicetemperatures, the steam pressure generated from vaporization of residualmoisture trapped within the casing may result in bursting of theexterior casing.

The present invention provides a product which is well adapted to beprefabricated by in-plant manufacturing techniques and which includes acast, cured and dried one-piece insulative covering for the pipe,offering economic and other advantages over the conventional techniqueof on-site assembly of insulation half shells, and provides a method forthe manufacture of the same. In one aspect, the invention consists in amethod of fabricating an insulated pipeline section comprisingpositioning within a tubular casing a mandrel in spaced relationshipfrom the interior surfaces of the casing, injecting into the spacetherebetween a liner composition comprising lightweight expandedaggregate in a foamed aqueous portland cement binder paste, curing theliner composition to a desired degree of final cure without permittingthe cured liner to bond to the mandrel and while maintaining at least apredetermined moisture content in the composition, withdrawing themandrel from the liner and casing, whereby the inner surface of theliner is exposed, drying the cured liner by subjecting its exposed innersurface to a drying atmosphere, and introducing into the cured and driedliner a pipe having its exterior conforming to that of the mandrel,whereby the inner surface of the liner closely conforms to but is indisjunction from the exterior surface of the pipe.

In the preferred form, the mandrel employed in the step of moulding theliner composition within the casing is the length of pipe that is to beemployed in the fabrication of the pipeline section. Thus, in thefabrication of the pipeline section, a length of pipe is introduced intothe casing to define the mould cavity, is withdrawn prior to the dryingoperation, and the same length of pipe is subsequently re-introducedinto the cavity defined by the inner surface of the dried liner.Preferably, the liner is permitted to cure to a desired degree of finalcure before the mandrel or pipe is withdrawn from the liner and theinner surface of the liner is exposed.

Bonding of the mandrel or pipe to the liner composition may be avoidedby coating its outer surface with a cement cure retarding agent so thatthe main portion of the liner composition may cure to the desired finalstage, and the pipe or mandrel may be withdrawn, before the portionsadjacent to and affected by the retarding composition set upsufficiently to bond to the mandrel or pipe. Alternatively, thecomposition may be permitted to partially cure and then the mandrel orpipe may be debonded by displacing it relative to the liner compositionfollowing which the liner composition is allowed to cure to its desiredfinal stage.

The mandrel or pipe may be debonded by displacing it relative to theliner composition at a stage at which the liner composition has set upsufficiently to prevent it from rebonding firmly to the exterior surfaceof the mandrel or pipe, but before the bond strength is sufficient topresent risk that displacement of the mandrel or pipe will result indisruption of the somewhat soft liner structure.

The method of the invention has numerous advantages. The exposure of theinterior surface of the liner greatly facilitates rapid drying of thecured liner and permits the drying operation to be conducted at elevatedtemperatures, for example well above the boiling point of water, thuspermitting the cured liner to be dried to a substantially completely drycondition in relatively short times. The outer casing, to which theliner composition remains bonded, may serve the dual function of formingan outer mould for the liner composition and of providing aweatherresistant and protective covering for the insulation material. Inconstruction of the the pipeline, the ends of successive pipe lengthswill be welded together to form an integral line, and it will normallybe desirable to secure the outer casing to supporting structures or, inthe case of underground installations, to have the pipeline casingremain stationary relative to the surrounding earth. In the productobtained with the present method, the pipe remains unbonded to the linerand so longitudinal thermal expansions and contractions of the pipelinecan be accommodated by the pipe sliding freely longitudinally within theliner.

In accordance with a further aspect, the invention consists in aprefabricated thermally insulated pipeline section comprising an outertubular casing, a pipe disposed within and in spaced relationship to theinner surfaces of the casing, and disposed between the casing and thepipe a one-piece cast thermally insulative liner comprising particles oflightweight expanded aggregate in a foamed, cured, and dried portlandcement binder, the liner bonding to the inner surfaces of the casing andhaving an inner annular surface closely conforming to but in disjunctionfrom the exterior surface of the pipe.

An insulated pipeline section in accordance with the invention and itsmethod of fabrication will now be described in more detail, by way ofexample only, with reference to the accompanying drawings, in which:

FIG. 1 is in the nature of a flow sheet illustrating partiallyschematically successive stages of a fabrication method;

FIGS. 2 and 3 are partial side views, partly in section and on anenlarged scale, of end portions of the casing and pipe sections employedin the fabrication method and circled at 2 and 3, respectively, in FIG.1;

FIG. 4 is a view, partially in section, on an enlarged scale of thecentral area of the pipeline section during the stage of injection ofthe liner composition, as circled at 4 in FIG. 1;

FIG. 5 is a view, partially in section, of the area circled at 5 in FIG.1, showing the casing and liner during the drying operation; and

FIG. 6 is a side view, partially in section, of the area circled at 6 inFIG. 1, showing the pipeline section having a sealing and protectiveplastic jacket over its exterior surface.

Referring to the drawings, in which like reference numerals indicatelike parts, FIG. 1 shows a length of pipe 10, e.g., steel pipe, to beinsulated, and a tubular outer casing 11 which will serve to retain theinsulation material. Preferably the casing 11 is moisture-impermeableand corrosion-resistant. In the preferred form, as shown in FIG. 2, thecasing 11 is formed from galvanized steel strip helically wound andunited at its edges by interlocking edge seams 13. The casing 11 issomewhat shorter than the length of pipe 10, so that in the finalproduct the pipe 10 protrudes beyond the casing 11 and the appliedinsulation material at each end to provide pipe end portions which willbe available for welding to those of adjacent insulated pipelinesections. Over each end of the pipe 10 may be fitted a cylindricalsleeve 14, preferably of metal, e.g., steel, with each sleeve 14 beingpositioned so that its end will be approximately in register with theadjacent end of the casing 11. The sleeves 14 have an internal diameterslightly larger than the external diameter of the pipe 10. Each sleeve14 is corrugated adjacent its inner end to form an inwardly directed rib16 which lightly grips the exterior of the pipe 10 so that the sleeve 14is easily rotated about or slid along the pipe 10. The rib 16 forms withthe pipe 10 a flow restriction which will reduce or prevent flow of theinsulation material slurry into the space between the sleeve 14 and thepipe 10.

The pipe 10 is inserted into the casing 11, and the pipe 10 and casing11 are supported so that the pipe 10 is approximately concentric withthe casing 11 by support means (not shown).

The next stage in the fabrication method, as illustrated in FIG. 1, isthe injection of a liner composition of a foamed aqueous slurry mixtureof lightweight aggregate and portland cement into the spacing or mouldcavity defined between the pipe 10 and the casing 11. In the case inwhich the pipeline section is to be provided with a sealing protectivejacket such as a jacket of thermoplastic resin, for example for use inunderground pipeline installations, the foamed slurry may be injectedthrough an aperture 17 in the casing 11. The foamed slurry, prepared ina mixing and foaming apparatus 18, is pumped by a pump 19 through a hose20. The foamed slurry of the liner composition 21 flows along the cavityand displaces air from the cavity. In the example shown in more detailin FIG. 4, the free end of the hose 20 extends into the annular cavityformed between the pipe 10 and the casing 11. The hose 20 may beprovided with an annular sealing gasket 22 forming a seal with the outersurface of the casing 11 adjacent the aperture 17 to prevent or reduceloss of the slurry composition 21. The hose 20 and gasket 22 may beretained on the casing 11 by a strap or the like (not shown) so that thepressure of the composition 21 does not displace the hose 20 outwardly.In cases in which the application of a sealing protective thermoplasticor other jacket over the casing 11 would be undesirable orinappropriate, as for example would normally be the case in pipelinesections intended to be employed in above-ground installations, it willnormally be desired to employ an imperforate outer casing 11 and, insuch cases, the foamed slurry mixture may be injected into one end ofthe casing 11 through a hose positioned as indicated by the broken line20a in FIG. 1.

The foamed slurry composition 21 consists of cement, expandedlightweight aggregate, and water, with or without such additives assmall quantities of fibres, air-entraining agents, and cement cureaccelerating agents. Preferred compositions are described in more detaillater herein.

In the preparation of the slurry mix and forming it into a foam, asillustrated in FIG. 1, the ingredients such as cement powder from hopper29, lightweight expanded aggregate from hopper 30, water from holdingtank 31, fibres from hopper 32, liquid surfactant or otherair-entraining agent from holding tank 33 and liquid cure acceleratingagent from holding tank 34 are metered into the mixing and foamingdevice 18 which may be in the form of an open topped trough 36 equippedwith helical stirring blades or paddles 37 supported on and rotated by ashaft 38 driven by motor 39. In the mixing and foaming device, theingredients are well mixed to form a slurry which, with continued mixingand agitation, becomes whipped up into an air-entraining foam. It isdesired to employ a foam containing a high proportion of entrained airbubbles in order to achieve a light weight set and cured productcontaining numerous interstices thus imparting low density and highthermal insulation values to the product. The wet density of the foamedslurry will desirably be in the range about 30 to about 60 lbs. per cu.ft., more preferably about 35 to about 45 lbs. per cu. ft. Typically,the foamed mix will have the consistency of shaving foam.

Before commencing the injection operation, plugs or end inserts ofinsulation material may be introduced into one or both ends of the mouldcavity defined between the pipe 10 and the casing 11. The end insertsmay consist of precast and cured, but not necessarily dried, cylindricalor semi-cylindrical shells of the same foamed composition as that usedfor the liner composition. Where used, the end inserts will be formedwith internal and external diameters conforming to the external diameterof the pipe 10, or of the sleeves 14 where employed, and the internaldiameter of the casing 11, respectively, and may be positioned withtheir end faces flush with the edge of the casing 11. In the case inwhich the injection is made through an aperture in the casing 11, asshown for example by the injection hose 20 in FIG. 1, a set of theabove-mentioned end inserts may be placed in each end of the mouldcavity before commencing the injection. The inserts, although conformingapproximately to the surfaces of the casing 11 and pipe 10, or sleeves14, provide small gaps, orifices or discontinuities permitting air to bedisplaced from the interior of the mould cavity during the injectionoperation. Where the injection is conducted from one end of the mouldcavity, as indicated by the injection hose 20a in FIG. 1, a set of theabove-mentioned end inserts may be placed opposite the point ofinjection, and, once the mould cavity has been filled and the injectionhose 20a withdrawn, a set of the end inserts may be placed within theopen end of the mould cavity, and pressed into contact with the injectedfoam composition within the cavity, thus displacing small quantities ofthe foam material through the gaps between the casing 11, pipe 10, andthe surfaces of the inserts as these are introduced.

Once the space or mould cavity defined between the pipe 10 and casing 11is completely filled with the foamed cementitious composition and theinjection hose 20 or the hose 20a have been removed, the injectedcomposition is permitted to cure. It is preferred to cure thecomposition at elevated temperature, as this greatly reduces the overallprocessing time, and, further, curing at elevated temperature tends toresult in products having greater strength characteristics than thoseobtained with comparable compositions cured at ambient temperature.Moreover, with the preferred compositions, wherein the contents of waterand of cement paste binder are relatively low, there is the risk that ifthe composition were permitted to undergo cure over a prolonged periodat ambient temperature, there would be a tendency for the cement binderpaste to drain away from the aggregate, resulting in a product withundesirably reduced strengths and structural non-uniformities. Duringthe curing operation, it is important to ensure that as far as possible,there is no loss of water or water vapour from the injected cementitiouscomposition in order to ensure that a sufficient quantity of waterremains in the material to allow the hydration reaction of the cement tobe accomplished. This can be achieved by maintaining the assembly of thepipe 10, the casing 11, and injected liner composition 21 throughout thecuring operation under a humidified atmosphere, preferably of at leastabout 95% relative humidity, more preferably at least about 97% relativehumidity.

Before permitting or commencing curing, quantities of the cementitiousfoam remaining on the exterior of the casing 11 adjacent the centralinjection aperture 17 and on the outer surfaces of the pipe 10 or casing11 adjacent their ends may be wiped away. Alternatively, any residues ofthe foam may be trimmed or scraped off at a subsequent stage before thecomposition has completely set up or cured.

In the case in which the above-mentioned end inserts are employedbetween the ends of the casing 11 and the adjacent surfaces of the pipe10, any tendency for loss of moisture from the injected compositionduring curing may be further reduced by placing an annular end sealingcap over each exposed end of the pipe 10, with the inner sides of theend caps being brought into close engagement with the annular end facepresented by the assembly of inserts and with the adjacent circular edgeof the casing 11.

In the preferred form of the curing operation, as illustrated in FIG. 1,the assembly of the pipe 10, liner 21 and casing 11 is placed in ahumidified oven, indicated somewhat schematically in FIG. 1 by anenclosure 41 and heating elements 42. As noted above, desirably theinterior of the enclosure 41 is maintained at an elevated relativehumidity of about 97%. In the curing oven, the assembly is exposed to anelevated curing temperature preferably in the range about 60° to about95° C., more preferably about 70° to 90° C. Greatly elevatedtemperatures are to be avoided as the increased vapour pressuregenerated from the moisture within the mix may tend to disrupt theinternal structure of the liner composition. In the most preferred form,the curing is conducted at a temperature of about 80° C.

In order to facilitate subsequent withdrawal of the pipe from the linercomposition to expose the interior of the liner for drying purposes, itis desired to cure the liner composition without permitting thecomposition to bond to the exterior of the pipe 10. This can be achievedby coating the pipe 10 with a cement cure retarding and/or lubricantcomposition before inserting the pipe into the casing 11. The coatingcomposition may comprise any conventional cement cure retarding agent.Suitable examples include modified salts of hydroxylated carboxylicacids. Additionally or alternatively the coating composition contains alubricant, preferably a particulate solid lubricant such as talc, toprovide a slippery surface facilitating sliding of the pipe relative tothe cured liner composition as it is withdrawn from the liner. In orderto ensure that the coating remains adherent to the pipe followingapplication and during the step of injecting the liner composition, thecoating composition preferably includes a thickener or binder, such asan aqueous based latex cement. The coating composition may be brushed,sprayed or wiped onto the pipe 10 to provide an even coating preferablyabout ten-thousandths to about one-sixteenth of an inch in thickness. Inthe case in which the coating composition contains no cure retardingagent, the lubricant composition prevents the liner composition frombonding to the pipe and permits the pipe to be withdrawn from the linerafter this has cured. When the coating composition contains a cureretarder, the main portions of the liner composition, remote from andunaffected by the retarding agent will set up to the desired degree ofcure but the portions adjacent the pipe do not set up sufficiently tobond to the pipe. The result is that in an interior annular zone of theliner adjacent the pipe, the cement paste does not set up and bondbetween the aggregate particles or to the pipe, leaving an annular layerof loose aggregate particles which are easily crushed or moved to oneside when the pipe is subsequently withdrawn. The depth of the zoneaffected by the retarding agent depends on the concentration of theretarding agent present in the retarding composition. It is desired tohave the insulating liner conform closely to the pipe so that the pipeis firmly supported within the liner and is not free to oscillate withinthe liner during handling or transportation, leading to risk of theweight of the pipe crushing the liner. Usually, the pipe is notperfectly cylindrical but will be out of round and will exhibit nonuniformities in its outside diameter at points along its length. Thesevariations and tolerances in the pipe diameter permit the pipe to lodgefirmly on reintroduction into the liner after the liner has been dried.The larger the pipe the greater the variations and tolerances indiameter, and the greater the depth of the zone of the liner that may bepermitted to be affected by the cure retarding agent. Thus, for examplein the case of a pipe of 20 inches OD the depth of the uncured zone maybe about one-quarter inch while in the case of a 2 inch OD pipepreferably this is no more than about one-sixteenth inch. Theconcentration of the cement cure retarding agent required in theretarding composition to produce an uncured zone of given depth in agiven liner composition may of course be readly determined by trial andexperiment.

Alternatively, the pipe may be mechanically debonded from the linerduring the curing step. In this case, the pipe 10 is debonded from theliner composition by displacing it relative to the liner composition ata stage of partial cure of the liner composition stage at which theliner composition has achieved a degree of cure sufficient to prevent itfrom rebonding to the pipe 10, but before the material is sufficientlystiff that displacement of the pipe relative to the liner would producecracks or other structural disruptions in the liner. The appropriateperiod of partial cure depends on the curing conditions, particularly onthe curing temperature, and on the nature of the composition of theliner composition and may, of course, in any given case be determined bytrial and experiment. Typically, the period of partial cure will beabout 2 to about 10 hours. With the preferred compositions and curingconditions, typically the debonding operation is conducted after apartial curing period of about 2 to about 5 hours. In the preferredform, the mechanical debonding operation is conducted by rotating thepipe 10 about its axis relative to the liner composition 21, casing 11and sleeves 14, if employed, for 2 or 3 turns. In the debondingoperation, the pipe 10 rotates freely within the sleeves 14, and doesnot disturb the anchoring or bonding of the sleeves 14 to the linercomposition 21. Normally, it will be convenient to remove the assemblyfrom the oven or other heated and humidified enclosure 41 during thedebonding operation. Following the mechanical debonding operation, theassembly, if it has been removed from the oven or other enclosure 41, isreturned thereto, and the curing operation is continued, desirably underconditions of elevated temperature and humidity, and conveniently underthe same conditions as those mentioned above.

Normally, it will be desirable to permit the liner composition toproceed to a fully cured hydrated condition in order to achieve aproduct having the maximum possible compressive strength obtainable withthe particular liner composition employed. Typically, the total periodof cure at elevated temperature, including any partial cure periodbefore mechanical debonding, will extend over about 8 to about 20 hours.With the preferred compositions and curing conditions the total periodof cure will be about 10 to about 15 hours in order to achieve asufficiently cured and hydrated product. As illustrated in FIG. 1, oncethe liner composition has achieved a desired stage of final cure, theassembly is removed from the oven or other curing enclosure 41, and thepipe 10 is withdrawn longitudinally from the interior of the casing 11and cured liner composition as indicated by the arrow 43 in FIG. 1, inorder to expose the interior surface of the cured liner, to facilitatedrying of the liner composition. As will be appreciated, the pipe 10 maybe withdrawn from the casing and liner composition at any stage at whichthe liner composition has achieved sufficient tensile strength andcoherency to render it self-supporting. Retaining the pipe 10 within theliner composition until this has finally cured limits the freedom ofwater to migrate from the liner composition, and thus helps retainwithin the liner composition a content of water sufficient forcompletion of the desired hydration reaction. Moreover, it is normallydesirable, for convenience of working, to remove the assembly from theconfines of the humidified enclosure 41 in order to withdraw the pipe10. Withdrawal of the pipe 10 at any stage before the liner compositionhas reached the desired stage of cure if performed outside the confinesof the humidified enclosure 41 and without waiting for the hot linercomposition to cool down could result in a gross loss of moisture fromthe liner composition with the risk that there may then be insufficientmoisture remaining in the composition to permit it to further hydrate tothe desired degree of cure.

Following the removal of the pipe 10, the casing 11, together with thecured liner 21 and the bonded in place sleeves 14 is placed within adrying oven, indicated in FIG. 1 by the enclosure 44 and the heatingelements 46, and is exposed to a drying atmosphere at elevatedtemperature, desirably above 100° C. in order to achieve drying within asatisfactorily short time, but preferably no more than about 250° C., inorder to avoid risk of cracking and shrinkage of the liner compositionthrough evolution of moisture at an unduly rapid rate. Where sealing endcaps have been applied over the ends of the pipe 10 and in engagementwith the end surfaces of the liner 21 and casing 11, the end caps are ofcourse removed before commencing the drying.

Typically, the drying will be conducted at a temperature of about 200°C. Usually, the drying will be conducted for a period of about 20 toabout 60 hours, sufficient to remove substantially all free water fromthe liner composition. More typically, the drying operation will beconducted for a period of about 50 hours. As illustrated in more detailin FIG. 5, during the drying operation the cylindrical interior surface47 of the cured liner composition 21 is freely exposed to the dryingatmosphere, thus readily permitting rapid drying of the cured liner to adesired degree of dryness.

Following the drying operation, the casing 11 together with the liner 21is removed from the drying oven 44 and the pipe length 10 is reinsertedwithin the liner, as illustrated in FIG. 1.

The above-described procedure may be modified by employing a mandrel, ofthe same external configuration as the pipe 10 which is ultimately to beused in the pipeline section, to form the inner wall of the mould cavitydefined within the casing 11 during the step of injection of the linercomposition. In such case, the mandrel, together with the sleeves 14 ifemployed, is positioned within the casing 11 before injecting the linercomposition, and, following curing of the composition, is withdrawn fromthe cured liner and, after drying, is replaced by the length of pipe tobe incorporated in the insulated pipeline section product. Thismodification is however, subject to the disadvantage that the mandreltends to become worn by abrasion through its contact with the curedliner, and is therefore not preferred.

In the case in which the casing 11 has been provided with an injectionaperture 17, this may be covered with a small patch, for example of anadhesive-backed plastic material, after the liner has been dried. Theentire exterior surface of the casing 11, including the exterior surfaceof the patch, is then coated with a protective and sealing jacket forexample by passing the complete assembly through a plastic coatingdevice, such as an extruder head 48 as shown in FIG. 1 to which acoating material e.g. molten thermoplastic is supplied along line 49,and wherein the exterior surface of the casing 11 receives a sealing andprotective jacket of thermoplastic resin or other material 51, as shownin more detail in FIG. 6. The coated or jacketed product thus obtainedis well adapted for use as an insulated pipeline section for use in theconstruction of an underground pipeline installation.

For above-ground insulated pipeline installations, where the use ofthermoplastic resin or other sealing and protective coatings is notnormally appropriate owing to their tendency to degrade under exposureto sunlight and weather, normally the casing 11 will be maintained as anintact, imperforate casing, with the liner composition having beeninjected at one end as indicated by the alternative injectionarrangement indicated by the broken line 20a in FIG. 1.

The resulting insulated pipeline sections, after welding together theexposed ends of adjacent pipe lengths 10 may have the welded sectionsinsulated by application of preformed half shell insulation sections,following which the half shell sections, after being united by strappingare covered with a sealing or protective corrosion-resistant cover e.g.a heat-shrinkable thermoplastic sleeve in the case of a below-groundinstallation or a preformed galvanized steel cover in the case of anabove-ground installation.

In the pipeline installation, the casings 11, liners 21 and sleeves 14,where employed, of the individual pipeline sections will normally beheld stationary relative to their surroundings as a result of thecasings 11 being fixed to support posts in the case of above-groundinstallations or being anchored in the surrounding earth in the case ofbelow-ground installations. As a result of expansion and contractionforces exerted on the pipeline, this will tend to reciprocate axiallyrelative to the liners 21 and casings 11 during service, with the resultthat the externally raised weld bead normally formed at the weld betweenadjacent pipe lengths 10 will tend to be displaced longitudinally to apoint at which it enters the liner 21 of the neighbouring pipelinesection. The sleeves 14 at the end of each section of preformedinsulative liner 21 offer the advantage that they preserve theinsulation material liner 21 from contact with the weld bead, thusavoiding risk of the weld bead abrading, crushing or fracturing thematerial of the liner 21 to the impairment of its insulative properties.

As regards the slurry compositions preferably employed in the presentmethod, desirably, the cement employed is of the high early strengthtype (ASTM type III) in order to reduce the time required followinginjection before the foamed slurry has set up to a degree permitting amechanical debonding operation to be carried out, and to reduce theoverall time required for the fabrication of the insulated pipelinesection. Desirably also, in order to reduce overall processing times,the mix will include one or more conventional cement cure acceleratingagents e.g. triethanolamine which when used will be present in an amountof about up to about 2% by weight based on the total weight of theslurry mixture, more typically about 1% by weight. Desirably, the amountof cement powder employed in the mix is no more than about 30% byweight, based on the total weight of the aqueous mixture. The set up andcured cement paste is a relatively poor thermal insulator, and atcontents of cement much above about 30%, the thermal insulationproperties of the cured composition are undesirably low. Use of amountsof cement of less than about 10% by weight tend to result in a productwhich has undesirably low strength properties, and which may haveinsufficent compressive and other strengths to permit it to be readilyhandled, stored or transported. More preferably, the cement content isabout 12 to about 20%, still more preferably about 15% by weight of thewet mix.

The lightweight aggregate material employed may be any expandedaggregate material having good thermal insulation properties and whichis resistant to the conditions of temperature to be encountered inservice and compatible with the cement material employed as the binderpaste. Examples of suitable materials include expanded perlite, expandedvermiculite, and foamed glass particles. By reason of its readyavailability, cheapness, and excellent thermal insulation properties,the use of expanded perlite is preferred. Desirably, the aggregateparticles employed have a diameter of no more than about one-eighthinch. The use of aggregate particles of much greater than this sizeresults in undesirably large inter-particle spacings in the mix and inthe cast slurry, thus resulting in the need for the use of anundesirably large amount of the aqueous cement paste binder in order toproduce a flowable slurry, and may give rise to problems in forming andmaintaining a foam of the required consistency from the mix. Desirably,the aggregate is employed in a range of about 10 to about 30% by weightbased on the total weight of the wet mix. The use of amount of aggregateof less than about 10 % by weight tends to result in the product havingundesirably low thermal insulation values, while use of amounts ofaggregate much in excess of about 30% by weight tends to result in aproduct which has insufficient strength properties. More preferably, theaggregate is present in an amount of about 12 to about 20%, still morepreferably about 15% of the wet mix. Typically, the amount of aggregatewill be such as to provide in the finally cured and dried linercomposition a content of about 25 to about 75%, more preferably about 30to about 50%, and still more preferably about 40% by weight, based onthe total weight of the composition

The slurry mix will preferably contain about 50 to 80% water by weight.The use of lesser weights of water tends to result in slurries ofundesirably stiff consistency which can not be readily fluidized andformed into a foam, while use of amounts of water greater than about 80%by weight may tend to result in products with undesirably low strengthcharacteristics. More preferably the water content is about 65% to about75% by weight of the wet mix.

Advantageously, in order to increase the flexural strengthcharacteristics of the cast and cured composition, the wet mix containsup to about 5% by weight of fibres, more preferably about 1% by weight.The use of a quantity of fibres much greater than about 5% by weight mayresult in the mix having an undesirably high density, and may give riseto problems in forming the mix into a foam. Desirably, the fibres are upto about 1 inch in strand length, more typically about one-half inchlength. The use of fibres of much greater strand lengths may createdifficulties in handling the slurry mix and forming it into a foam. Anyfibres that are compatible with the cement paste may be employed.Examples include glass fibres and synthetic resin fibres, such aspolyester fibres or fibrilated polypropylene. For reasons of its lowcost and excellent flexural strength imparting properties, the use ofglass fibres is particularly preferred.

It is desirable to include an air-entraining agent in the mix in orderto facilitate producing and maintaining the foam. Any conventionalair-entraining agent may be employed and preferably the agent will bepresent in an amount of up to about 1% by weight, based on the totalweight of the wet mix, more preferably from about 0.05 to about 0.25% byweight, and still more preferably in an amount of about 0.1% by weight.As a typical example of an air entraining agent may be mentioned theliquid surfactant material available under the trade mark CES-465 fromArmak Chemicals Ltd., Saskatoon, Sask.

As noted above, desirably the foamed mix will have a wet density ofabout 30 to about 60 lbs per cu. ft., more preferably about 40 lbs percu. ft.

Preferably, the finally cured and dried insulative liner compositionwill have a dry density of about 10 to about 30 lbs per cu. ft., morepreferably about 12 to about 20 lbs per cu. ft., and still morepreferably a density of about 15 lbs per cu. ft., and will have athermal conductivity of about 0.3 to about 0.8, more typically about 0.4to about 0.7 and more preferably about 0.5 BTU/inch/°F./hr/ft² at 400°F. mean, and a compressive strength of about 40 to about 200 psi, morepreferably at least about 100 psi.

EXAMPLE

In one example, the procedure described above with reference to thedrawings was followed employing a liner composition as follows:

    ______________________________________                                        Ingredients             % by weight                                           ______________________________________                                        Portland cement (ASTM Type III)                                                                       15                                                    Expanded perlite (less than 1/8" diameter)                                                            15                                                    Glass fibres (1/2" length)                                                                            1                                                     Armak CES-465 (air entraining agent)                                                                  0.1                                                   Triethanolamine         1                                                     Water                   balance                                               ______________________________________                                    

The mix was agitated to yield a foam of wet density about 40 lbs per cu.ft. Curing was conducted at 80° C. and 97% RH for 13 hours, withmechanical debonding after 3 hours by rotation of the pipe. Afterwithdrawal of the pipe, the assembly was dried at 200° C. for 48 hours.The cured and dried liner had a dry density of 16 lbs per cu. ft., acompressive strength of 100 psi, and an insulative value of 0.5BTU-in/°F./hr.ft² at 400° F. mean.

We claim:
 1. Method of fabricating an insulated pipeline sectioncomprising positioning within a tubular casing a mandrel in spacedrelationship from the interior surfaces of the casing, injecting intothe space therebetween a liner composition comprising lightweightexpanded aggregate in a foamed aqueous portland cement binder paste,curing the liner composition to a predetermined degree of cure withoutpermitting the cured liner to bond to the mandrel and while maintainingat least a predetermined moisture content in the composition,withdrawing the mandrel from the liner and casing, whereby the innersurface of the liner is exposed, drying the cured liner by subjectingits exposed inner surface to a drying atmosphere, and introducing intothe cured and dried liner a pipe having its exterior conforming to thatof the mandrel, whereby the inner surface of the liner is contiguouswith but is in disjunction from the exterior surface of the pipe. 2.Method as claimed in claim 1 wherein said pipe is employed as themandrel.
 3. Method as claimed in claim 1 wherein the step of curing theliner composition comprises partially curing the liner composition,debonding the mandrel by displacing it relative to the partially curedliner composition, whereby the liner composition remains adherent to thecasing, and permitting further cure of the liner composition to a saidpredetermined degree.
 4. Method as claimed in claim 3 wherein thepartial curing is conducted for about 2 to about 10 hours before themandrel is debonded.
 5. Method as claimed in claim 4 wherein the partialcuring is conducted for about 2 to about 5 hours.
 6. Method as claimedin claim 3 wherein the displacing comprises rotating the mandrel aboutits axis.
 7. Method as claimed in claim 1 wherein prior to the injectionof the liner composition, the exterior of the mandrel is coated withcement cure retarding composition, and the curing step comprises curingthe liner composition for a period sufficient to cure the main portionof the composition, the retarding composition preventing the portion ofthe liner composition adjacent the mandrel from setting up sufficientlyto bond to the mandrel.
 8. Method as claimed in claim 7 wherein theretarding composition comprises a paste formed from a liquid cureretarding agent and a thickener therefor.
 9. Method as claimed in claim8 wherein the thickener is an aqueous-based latex cement.
 10. Method asclaimed in claim 7 wherein the cure retarding composition comprises alubricant.
 11. Method as claimed in claim 10 wherein the lubricantcomprises talc.
 12. Method as claimed in claim 1 wherein prior to theinjection of the liner composition, the exterior of the mandrel iscoated with a lubricant composition which prevents the line compositionfrom bonding to the pipe and permits the pipe to be withdrawn from thecured liner.
 13. Method as claimed in claim 12 wherein the lubricantcomposition comprises a paste formed from a liquid binder and aparticulate solid lubricant material.
 14. Method as claimed in claim 13wherein the liquid binder is an aqueous based latex cement.
 15. Methodas claimed in claim 1 wherein the liner composition is cured bymaintaining it at a temperature of about 60° to about 95° C. and underan atmosphere of relative humidity of at least about 95%.
 16. Method asclaimed in claim 15 wherein said temperature is about 70° to about 90°C.
 17. Method as claimed in claim 16 wherein said temperature is about80° C.
 18. Method as claimed in claim 15 wherein the relative humidityis at least about 97%.
 19. Method as claimed in claim 1 wherein theliner composition is subjected to a total curing period of about 8 toabout 20 hours.
 20. Method as claimed in claim 19 wherein said totalperiod is about 10 to about 15 hours.
 21. Method as claimed in claim 1wherein the liner composition is cured to a fully cured and hydratedstate before drying.
 22. Method as claimed in claim 1 wherein themandrel or pipe is retained within the liner until the liner compositionhas fully cured.
 23. Method as claimed in claim 1 wherein before theinjection of the liner composition a cylindrical sleeve is fitted overeach end of the mandrel with its outer end approximately in registerwith the adjacent end of the casing, the sleeve extending around themandrel with sufficient clearance to permit rotational and axialmovement of the mandrel relative to the sleeve, the outer surface of thesleeve becoming bonded to the liner composition.
 24. Method as claimedin claim 23 in which the sleeve has adjacent its inner end an annularrib extending radially inwardly and lightly engaging the outer surfaceof the mandrel, to restrict flow of the liner composition between thesleeve and the mandrel.
 25. Method as claimed in claim 1 wherein thecured liner is dried to eliminate substantially all free moisturetherefrom.
 26. Method as claimed in claim 1 wherein the drying isconducted at a temperature of about 100° to about 250° C.
 27. Method asclaimed in claim 26 wherein the temperature is about 200° C.
 28. Methodas claimed in claim 1 wherein the injected liner composition comprisesby weight about 10 to about 30% cement, about 10 to about 30% of theaggregate, about 50 to about 80% water, 0 to about 5% by weight fibresof up to about 1 inch strand length, 0 to about 1% of an air entrainingagent, and 0 to about 2% by weight of a cure accelerating agent, and hasa wet density of about 30 to about 60 lbs per cu. ft.
 29. Method asclaimed in claim 28 wherein the composition contains by weight about 12to about 20% cement, about 12 to about 20% of the aggregate and about 65to about 75% water.
 30. Method as claimed in claim 28 wherein thecomposition contains about 1% by weight of said fibres.
 31. Method asclaimed in claim 30 wherein the fibres are about one half inch in strandlength.
 32. Method as claimed in claim 28 where the composition containsabout 0.05 to about 0.25% by weight of the air entraining agent. 33.Method as claimed in claim 32 wherein the air entraining agent ispresent in an amount of about 0.1% by weight.
 34. Method as claimed inclaim 28 wherein the composition contains about 1% by weight of a cureaccelerating agent.
 35. Method as claimed in claim 28 wherein the wetdensity of the composition is about 35 to about 45 lbs per cu. ft. 36.Method as claimed in claim 1 wherein the liner composition is injectedthrough an aperture in the side wall of the casing intermediate itsends, and including the steps of closing the aperture with a patch, andsealing the exterior of the casing after curing and drying of the linerby applying a sealing jacket.
 37. Method as claimed in claim 36 whereinthe sealing jacket comprises a coating of a thermoplastic resin. 38.Method as claimed in claim 1 wherein the liner composition is injectedfrom an end of the casing.